Bio-oils produced by fast pyrolysis of biomass offer a potentially inexpensive and renewable source for liquid fuels. However, raw bio-oils are complex chemical mixtures with a high oxygen content, typically near 40 wt %, which can lead to a number of undesirable characteristics including low heating values compared to conventional fuels, incompatibility with conventional fuels, chemical instability, and high acidity due to carboxylic acid functionalities. Carboxylic acids include up to 25 wt % of bio-oils, with acetic and formic acids making up the majority of this fraction. The acidity of these compounds contributes to the degradation and instability of bio-oils. Therefore, reducing the acid content is important both for improving fuel properties and fuel stability. Consequently, catalytic upgrading of bio-oils is a critical requirement to reduce the oxygen content of bio-oils and generate a liquid product with “drop-in” ready fuel properties.
Upgrading of bio-oil vapors directly after pyrolysis offers a number of potential advantages, such as yielding a stabilized product upon condensation. However, effective deoxygenation of bio-oils requires catalysts that can readily activate H2, are active in low hydrogen-to-carbon environments, and are stable under acidic conditions.
Solid metal phosphide catalysts have the potential to meet these criteria. However, the methods typically employed to produce solid metal phosphide catalysts require high reaction temperatures and/or air-sensitive reagents that can be cost prohibitive. In addition, current methods sometime produce particles with less than satisfactory physical properties. In some cases, current methods for synthesizing solid metal phosphide catalysts result in inhibitory oxide layers coating the outer surface of the catalyst. Many current methods may also yield polydisperse phases with varying shapes, sizes, and compositions. Such variability can result in downstream manufacturing issues such as bed-life, catalyst activity, reactant conversion rates, and product yield. Finally, current synthesis methods often result in hollow metal phosphide particles. Hollow particles are believed to increase the variability of active sites and provide shorter catalyst life due to the mechanical attrition of the solid catalyst, especially in fluid-bed reactors. These all result in higher bio-oil upgrading costs.
Thus, there is a need for improved methods for producing solid metal phosphide catalysts that result in better quality and better performing metal phosphide materials that will ultimately perform better at the catalytic upgrading of bio-oils.